When using Selenium Grid, generally, you should start a hub first because nodes are dependent on that. To start a hub with default parameters, run the following command from a command-line shell or console. This will work on all the supported platforms, windows linux, or MacOS. Let's say we have downloaded version 2.39.0 verion:
java -jar selenium-server-standalone-2.39.0.jar -role hub
This starts a hub using default parameter values.